There is a clear demand for the monitoring of air-borne compounds that can have health effects on exposed individuals. A great interest exists for compounds that have occupational exposure limit values, set by governmental bodies, to ensure that the levels of such compounds are satisfactory low. In many cases, it is not known what the air contaminants consist of and for this reason, it is of interest to learn more details about the nature of these “unknown” compounds and to reveal the identity of the most predominate ones. Another field of interest is to study and check the effect of measures with a view to reducing these levels in air, e.g. to check the “true” ventilation efficiency or other measures to control the air levels. Devices for this purpose can also be used for the monitoring of the quality of compressed air and air in respiratory protective devices. Other fields of application for such devices are e.g. the control of different volatile compounds present in food. Such compounds can be used as markers for degradation of certain food components or to monitor raw materials to ensure a satisfactory quality. Such devices may also be used to ensure that other compounds have not contaminated to food. In hospitals, such devices can be used to check the air levels of e.g. narcosis gases and to ensure that the personnel, patients or others are not exposed to toxic levels. Chemical warfare agents are compounds that need to be checked for in order to reveal the presence thereof and to ensure that individuals are not exposed.
In environmental analysis there is a need to monitor the quality of air in cities, public places and in the nature. One purpose is to obtain background data for statistical studies and to check if the levels are below the levels set by national and international bodies. They can also be used to check if the emission of industrial pollutants results in exposure in the nature or in populated areas. The achieved data can have an impact on decisions and interpretation of a certain situation. There is therefore a demand of a satisfactory high quality of the data.
There are many examples of air pollutants that occur in both gas and particle phase. Of special interest are the size fractions that have the ability to reach the lower respiratory tract. There are reasons to believe that the toxicology is different depending on not only the chemistry as such but also on the distribution on different target organs in the body of humans. There is a need to know more about the exposure to the respirable particle fraction present in air.
Numerous devices exist for the monitoring of air-borne compounds and there is a great variety of technology used. In principle, the devices can be grouped in selective and non-selective devices. Non-selective devices give a response for several compounds and do not differentiate between two or several compounds and may also result in false positive results. Such devices are today still used, possibly due to the low cost. In many applications, false positive results can give rise to a high cost for the user, if costly measures are performed from invalid data.
Selective devices give a certain response for a selected compound or a group of compounds. Other present compounds do not interfere with the result. The frequency of false positive results will be much less as compared to non-selective monitoring. The quality of the data obtained is essential. Typical factors that describe the quality of the data are: repeatability, reproducibility, linearity (calibration graph characteristics with intercept and background), detection limit and quantification limit. In addition, knowledge regarding the interference from other compounds is necessary. It needs to be mentioned that a certain compound can influence the result even if the compound does not itself give rise to a response.
Similar techniques for the detection of air-borne compounds involves the use of e.g. photo ionisation detectors (PID, Thermo Scientific, Franklin, Mass., USA), flame ionisation detectors (FID, Thermo Scientific, Franklin, Mass., USA), infrared detectors (IR), portable gas chromatography (GC)-PID (PID Analyzers, Pembroke Mass., USA), portable GC-mass spectrometers (MS, Inficon Inc., New York, USA), GC-DMS ((Differential Mobility Spectrometry), Sionex Inc., Bedford, Mass., USA). All techniques give a response for a certain analyte, but to know the concentration the response need to be translated to concentration by using information from a more or less sophisticated calibration curve. For many of the above techniques, the response varies with time due to ageing, contamination of the detector (reduces the signal) and other variables.
The GC-DMS technique mentioned above is used in the MicroAnalyser instrument (Sionex Inc., Bedford, Mass., USA). The GC-DMS technique is based on GC separation, with regards to compound volatility, in combination with the separation in a DMS sensor, with regards to other molecular properties such as size shape, charge etc.
There are several drawbacks with the present types of instruments. For PID and FID, identification of the individual chemicals is not possible. PID and FID detectors measure the sum of VOC (Volatile Organic Compounds). Infrared detectors suffer from problems with inferences. IR detectors are not possible to use when monitoring VOCs at low concentration when other interfering compounds are present.
For direct monitoring using GC-PID (e.g. VOC71M from Environment s.a.; www.environnementsa.com) and the GC-DMS instrument (e.g. Sionex Inc., Bedford, Mass., USA) there are limitations leading to inaccurate identification and quantification of analytes, and external complementary pre or post-calibration have to be made. For the existing products it is not possible to perform calibration automatically in the field. Further, there are problems with the occurrence of a non-linear relation between the sampling time and determined concentrations, which thereby disables long time sampling if the amount exceeds the calibration range. Further, when a volume is collected it needs to be calibrated to a volumetric volume and possibly corrected for the ambient temperature and air pressure. The sampling of a volume in a certain sampling volume container or on a sorbent followed by thermal desorption (in the case of a sorbent) and thereafter injecting the collected compounds on the GC the chromatographic peaks will be broadened in a way that the resolution of the chromatography will be affected.
Another problem in known techniques is analysing different analytes with a great difference in concentration. Compounds that have been introduced to the sampling system cause carry over problems and memory effects to samples that are analysed. In fact, there are no practical means to ensure that the estimated concentration is true if not a sample that represents the baseline or the background or the blank is analysed before and after the real sample from the environment is collected.
Another important parameter in this area is the gas flow containing the compound to detect, i.e. the analyte, in the apparatus used for the detection. During the sampling of compounds in air it is of importance to be able to control and log the flow and volume of the acquired amount of air through the sampling device as there is a direct correlation between the contents in a sample and the air volume collected. Taking several samples simultaneously is also of importance for three reasons, more precisely for increasing the accuracy of a certain sample, for detecting erroneous samples and for acquiring different compounds simultaneously. When handling sampling results, it is also important to be able to track how the sample was collected, the time, the flow, the temperature, the pressure and the humidity.
Existing solutions to maintain a stable flow during sampling do not prove to maintain a stable flow over time and requires field calibration. The flow speed needs to be calibrated before and after sampling to ensure that the sampling speed is correct and have not changed over time. A logging functionality is also often missing.
An existing solution tried is the SKC AirChek pump (see www.skcinc.com), where a differential pressure sensor indicates if a change in the flow system back pressure has occurred, and adjusts the pump control signal to compensate for this. However, this solution has proven to give drift errors over time, and a calibration with an external flow meter is required in order to set a certain flow rate of its pump.
Another existing solution is the Casella Apex pump system (see www.casellameasurement.com). It has a logging function, an ability to transfer logged data to a PC, and ability to control flow via a display and buttons. The inventors behind the present invention have conducted tests on these pumps in 2006, and the results did not concur with its specifications, as the pumps did not manage to keep a stable flow as a sampler inducing a certain backpressure was attached to it. For samplers with high backpressure, the Casella Apex did not work at all.
A problem with existing pump systems is that the flow sensors incorporated in them may fluctuate with the temperature of flow sensor electronics. Most flow sensors, using different techniques for the actual measurement of gas flow, have an output voltage signal corresponding to the measured flow. The output signal is however easily affected by the temperature of the electronic components in the flow sensor.
A further problem in existing apparatuses for the detection of air-borne compounds is the occurrence of a memory effect in the system in view of different analyte compounds and also other compounds of no interest to detect which have passed through the system. This phenomenon gives rise to inaccurate and erroneous detection results. The instrumentation is in most cases fully flexible and a tubing need to be connected from the measuring spot to the instrumentation. The tubing can in many cases be long and contains a certain volume. To get representative samples to be introduced into the instrument and the sampling device the volume needs to be flushed with several more volumes as compared to the volume of the tubings.
In view of this, there is a great demand for an improved direct monitoring device for the detection of air-borne compounds or analytes and for an improved method for the detection of such compounds or analytes.
There is further also a great demand for an improved pump for monitoring devices for the above mentioned detection of air-borne compounds, a pump that has the ability to deliver adequate pumping performance required for accurate measurements.